A. Field of the Invention
This invention relates to the field of power line carrier current telephone extension systems. More particularly, this invention relates to secure system control protocols for line carrier telephone extension systems, which provide multiple extension arbitration logic, privacy, security and other telephone signaling options, which technique is called Secure Access (SA).
B. The Background Art
Numerous attempts have been made to utilize the power lines of buildings for telephone communications with varying degrees of success. Line carrier telephone extension systems utilize 1) a single BASE unit per central office line (for interfacing line carrier signals with the central office line) and 2) one or more EXTENSION units, one for each extension phone set (for interfacing said line carrier signals with said extension phone sets). Full duplex communication is usually attempted by using two carrier frequencies, one for each direction. Usually a transmitter and receiver are included in each base and extension station which are operating simultaneously when communication is in progress. For economic reasons FM modulation of the carrier by audio (speech) signals is often employed, of which U.S. Pat. Nos. 3,949,172 and 4,701,945 are examples. The physical range of line carrier communications systems is usually limited to the confines of the secondary of the power line transformer and is subject to severe noise and attenuation variances depending upon carrier frequencies and load conditions.
While noise and attenuation problems may be overcome somewhat by increasing transmitter power levels or increasing receiver sensitivity, this increases the potential for interference from neighbors with similar systems, which aggravates the problems of privacy and security prior art line carrier systems exhibit. Thus, privacy or security breaches arise when other users on the same power distribution system or in the same apartment complex with the same line carrier devices can make long distance calls on another user's line and eavesdrop on their conversations. But people driving by in cars cannot scan and eavesdrop on the line carrier signals the way they can cordless phone RF transmissions. The impact of these problems has already been demonstrated in the cordless telephone industry, which shares some of the same limitations as the line carrier industry. Security from line capture by third parties has been provided by security coding protocols in the cordless phone industry. But people with scanners or cordless phone receivers can eavesdrop on a neighbor's cordless call. Also, no provision is made for multiple cordless extension phones utilizing the same base. Similarly, in line carrier systems it was not permitted for more than one system to use the same power lines in the same building.
The subject matter of the instant invention relates somewhat to that of a previous application by the inventors hereof filed Oct. 7, 1991, Ser. No. 07/773,009 now U.S. Pat. No. 5,319,634, entitled "MULTIPLE ACCESS TELEPHONE EXTENSION SYSTEMS AND METHOD". The previous application relates primarily to a combination of radio frequency (RF) transmission techniques such as frequency division multiple access (FDMA) and code division multiple access (CDMA or spread spectrum). The instant application relates to the use of audio and subaudio frequency system control logic keys which unlock telephone signalling, RF and voice transmissions such as capturing the central office line, ringing the extension and speaking in full duplex between the base and extension units. The use of system control logic messages with security keys allows for non-simultaneous multiple use of the same power distribution system with privacy from eavesdropping and security. The system control logic also prevents another user from interrupting and terminating a call which is in progress. Other multiple extension arbitration logic functions are provided to permit one extension to put a call on hold and transfer the call to another extension unit. Use of FDMA in combination with security/control keys provides true multiple access and security at a low cost.
Commercial applications for the secure access line carrier telephone; extension technology include computer modem interfaces for pay-per-view (PPV) systems and gas pump/security office systems, where it is difficult to retrofit phone lines and twisted pairs into existing construction.
Since an increasing number of people live in apartments and condominiums which share power lines and because an increase in transmission power and receiver sensitivity are required to overcome attenuation and noise problems, it is important for a telephone extension system to utilize privacy and security coding in a unique and skillful manner to permit acceptable operation.